Apparatus and Method of Use for a Top-Down Directional Solidification System

Abstract

This invention relates to an apparatus and a method of use for a top-down directional solidification system, such as for casting solar grade silicon. The invention includes a vessel with a pressure relieving device to prevent pressure build up in the liquid as the solidification front advances from a downward direction. The invention also includes a drain to remove impurities, such as the concentration in the remaining liquid phase as casting nears completion.

1 . A vessel for top-down directional solidification and purification suitable for casting silicon, the vessel comprising:
 a crucible; and   a pressure equalizing device.   
     
     
         2 . The vessel of  claim 1 , wherein the pressure equalizing device comprises a standpipe for equalizing a liquid pressure below a mass of solidified silicon. 
     
     
         3 . The vessel of  claim 2 , wherein the standpipe comprises a heated tube internal to the crucible. 
     
     
         4 . The vessel of  claim 3 , wherein the standpipe comprises one or more orifices disposed on or above a bottom of the crucible. 
     
     
         5 . The vessel of  claim 2 , wherein the standpipe comprises a heated tube external to the crucible with a p-trap. 
     
     
         6 . The vessel of  claim 1 , wherein the vessel comprises fused silica. 
     
     
         7 . The vessel of  claim 1 , further comprising one or more magnetic field coils to maintain a mass of solidified silicon on a mass of molten silicon. 
     
     
         8 . The vessel of  claim 7 , wherein the one or more magnetic field coils comprise boron nitride coated graphite. 
     
     
         9 . The vessel of  claim 7 , further comprising an ingot stabilizer. 
     
     
         10 . The vessel of  claim 1 , wherein the crucible comprises sliding walls movable with respect to a solidification front. 
     
     
         11 . The vessel of  claim 10 , wherein the sliding walls comprise silica. 
     
     
         12 . The vessel of  claim 1 , further comprising a drain to remove impurities. 
     
     
         13 . A method for top-down directional solidification and purification suitable for casting silicon, the method comprising:
 providing a molten feedstock in a vessel with a pressure equalizing device;   solidifying from a top direction by extracting heat through a top or at least one side of the vessel; and   reducing impurities within the vessel through a drain.   
     
     
         14 . The method of  claim 13 , wherein the reducing comprises flowing the impurities through the drain in or above a bottom of the vessel. 
     
     
         15 . The method of  claim 13 , wherein the reducing comprises activating a heater to melt at least a portion a solid feedstock. 
     
     
         16 . The method of  claim 13 , wherein the impurities comprise silicon carbide or silicon nitride. 
     
     
         17 . The method of  claim 16  wherein the impurities comprise particles greater than about 10 microns. 
     
     
         18 . The method of  claim 13 , further comprising equalizing a liquid pressure below a mass of solidified silicon. 
     
     
         19 . The method of  claim 18 , wherein the equalizing includes flowing molten silicon in or up a standpipe. 
     
     
         20 . The method of  claim 13 , further comprising contacting a top surface of the molten feedstock with a seed crystal. 
     
     
         21 . The method of  claim 20 , wherein the contacting excludes actions of rotating. 
     
     
         22 . The method of  claim 20 , wherein the contacting excludes actions of pulling upward. 
     
     
         23 . The method of  claim 20 , wherein the extracting heat includes removing at least a portion of the heat through the seed crystal. 
     
     
         24 . The method of  claim 20 , wherein the seed crystal comprises a shaped geometry adapted for cooling and initializing crystal growth. 
     
     
         25 . The method of  claim 13 , further comprising maintaining a mass of solidified silicon on a mass of molten silicon with while preventing liquid leakage using one or more magnetic field coils. 
     
     
         26 . The method of  claim 25 , further comprising holding the mass of solidified silicon with an ingot stabilizer. 
     
     
         27 . The method of  claim 25 , wherein magnetic field coils deactivate after a solidification interface passes. 
     
     
         28 . The method of  claim 13 , further comprising moving slideable walls down in tandem with the advance of a solidification front. 
     
     
         29 . The method of  claim 28 , further comprising equalizing a liquid pressure below a mass of solidified silicon, by allowing generally upward displacement of the mass of solidified silicon. 
     
     
         30 . A high purity silicon ingot made by the method of  claim 13 . 
     
     
         31 . The ingot of  claim 30 , wherein the ingot comprises primarily silicon selected from the group consisting of multicrystalline silicon, monocrystalline silicon, near monocrystalline silicon, geometric multicrystalline silicon, and combinations thereof. 
     
     
         32 . The ingot of  claim 30 , wherein the ingot is substantially free from radially distributed defects. 
     
     
         33 . The ingot of  claim 30 , wherein the ingot comprises a carbon concentration of about 2×10 16  atoms/centimeter cubed to about 5×10 17  atoms/centimeter cubed, an oxygen concentration not exceeding 7×10 17  atoms/centimeter cubed, and a nitrogen concentration of at least 1×10 15  atoms/centimeter cubed.